The invention relates to a blow molding having capacity of at least 250 ml, its shell being composed of polystyrene and being produced via injection stretch blow molding.
The invention further relates to the process for production of this blow molding and to the use of the blow moldings.
Polystyrene is rubber-free or rubber-containing polystyrene, styrene-butadiene copolymers, and also mixtures composed of the polymers and/or of block copolymers.
Rubber-free polystyrene is also termed GPPS (general purpose polystyrene).
Conventional rubber-containing styrene polymers comprise a rubber phase which is based on dienes and which has been dispersed in a hard matrix composed of styrene polymer. Impact-resistant polystyrene (HIPS, high impact polystyrene) thus comprises a hard polystyrene matrix and, dispersed therein, polybutadiene rubber particles, for example. It is obtained by first preparing a rubber—e.g. in solution—and dissolving the rubber in styrene, and then polymerizing the mixture to give the HIPS.
The term polystyrene also includes styrene-butadiene block copolymers.
Production of relatively small blow moldings via blow molding of polystyrene is known but is very little used. Examples are the 65 ml drinking-yoghurt containers from Yakult or 200 ml yoghurt pots with trade mark Landliebe® from the dairy company Campina. However, these blow moldings have a low blow-up ratio. Exposure to mechanical load can cause destruction of these containers through breakage resulting from splitting. Vessels with relatively high capacity which are safe to transport cannot be produced by this technique. The tendency of a vessel to split when dropped from a height of 1 meter, for example, rises extremely rapidly with volume/weight of contents.
JP 2005-145560 describes drinks containers obtainable via injection blow molding. The containers are produced with an impact-resistant polystyrene whose residual content of styrene is less than 100 ppm and whose total content of volatile organic substances is less than 150 ppm, which is prepared via an extraction process from polystyrenes whose residual contents are higher. The capacity of the molded containers is 138 ml. Articles having less than 1% rubber content cease to have desirable organoleptic properties.
JP 2005-225960 proposes using extrusion blow molding processes for production of relatively large containers, such as bath tubs. Here, high-molecular-weight HIPS whose average molecular weight is from 250 to 300 000 daltons and which have defined molar mass distribution are converted to blow moldings. The extrusion blow molding process is not suitable for production of blow moldings such as bottles, which have very precise geometries—in particular in the region of the screw thread and the closure of the blow moldings. The abovementioned high-molecular-weight HIPS has too little flowability for other processes, such as injection blow molding, and cannot therefore be used for this process.
For this reason, relatively large blow moldings, such as bottles, have hitherto been produced from other materials, in particular PET, PP, PVC, or PC. Production processes that have been developed for these articles are particularly injection blow molding and injection stretch blow molding. http://en.wikipedia.org/wiki/Blow_molding has information on injection blow molding and injection stretch blow molding.
The PET bottle, successful in the drinks sector, is less suitable as a container for dairy products which comprise living cultures, e.g. yoghurt, kefir, whey, etc. Other materials which have relatively high permeability to oxygen or carbon dioxide have unsatisfactory splitting behavior. Because these materials have hitherto provided insufficient safety during transport, they have hitherto been very little used as containers for hazardous products, such as corrosive liquids. Chemical resistance to materials such as strong bases is also inadequate.